This invention relates to an apparatus for and a method of reproducing information from a recordable optical disc on which information is partially recorded.
An optical disc, so-called compact disc (CD), has a diameter of 12 cm (approximately 4.72 inches) or 8 cm (approximately 3.12 inches). On a compact disc, as shown in FIG. 1, a lead-in area, a program area and a lead-out area are formed in turn from an inner circumference of the disc. Signal recorded onto the disc is an EFM (Eight to Fourteen Modulation) digital signal including main codes which are main information such as music information, and sub-codes such as time codes (Q-data) indicating time information. In the EFM signal, clock signal used for a spindle servo control is included. In the spindle servo control, the clock signal is compared with a reference synchronizing signal to produce a difference signal which is fed back to a spindle motor through an error amplifier, etc. and used for a rotational frequency control of the spindle motor.
On the lead-in area, an index information called TOC (Table of Contents) is recorded. The TOC includes, as sub-codes, information showing a total number of music pieces recorded on the program area and a total play time thereof are recorded. On the program area, the Q-data, a track number (TNO) indicating the number of music piece, relative play times (P-TIME) from the head of each track and a total play time (A-TIME) from a first track are recorded as sub-codes, in addition to music information recorded as main code. Further, lead-out codes indicating the lead-out area is recorded on the lead-out area.
The CD having the above recording format is a medium exclusively used for reproduction. Recently, it is known a recordable compact disc (R-CD) having the almost same recording format as the CD and on which information is additionally recordable. Though the recording format of the R-CD, generally called as "Orange Book", is in conformity with that of the CD, a Power Calibration Area (PCA) and a Power Memory Area (PMA) are further formed inner circumferential side of the lead-in area for convenience' sake of recording operation, as shown in FIG. 2 and 3.
On the recording tracks of the R-CD, guide grooves are formed which is wobbled at a frequency obtained by frequency-modulating a 22.05 KHz carrier by data showing absolute time information (ATIP: Absolute Time In Pregroove). A laser beam for recording information onto the disc is traced along the guide grooves under the tracking servo control. Further, the rotational frequency of the spindle motor controlled in such a manner that the center frequency of the wobbling becomes equal to 22.05 KHz. In this way, the tracking servo control and the spindle servo control for a disc on which no information is recorded (hereinafter referred to as "virgin disc") can be performed.
ATIP information has data of 42-bits per one frame which consists of a four-bit sync pattern (SYNC), three subsequent pieces of 8-bits data each indicating minute (MIN), second (SEC) and frame (FRAME) and a 14-bits error correction code (ECC).
Absolute time information indicated by ATIP information represents one second by 75 frames, same as the Q-data sub-code.
In addition to absolute time information, ATIP information contains coded special information, which is represented by a combination of three beginning one-bits (MSBs) of the data pieces respectively showing the "minute", "second" and "frame" (the fifth, thirteenth and twenty-first bits from the beginning of the frame). Particularly, when the combination of the fifth, thirteenth and twenty-first bits shows "1", "0" and "1", three bits consisting of the sixth to eighth bits show a recommended recording powder level (light intensity) for the disc. That is, eight recommended recording power levels, which are stepwise distributed, are defined in accordance with eight combinations of the sixth to eigth bits, namely, "000" to "111". The recommended recording power levels are defined for a condition that the wavelength of the recording beam equal to 780 nm, and the temperature T is equal to 25.degree.. However, in actuality, the optimum recording power level (light intensity) is not always equal to the recommended recording power level because the wavelength depends on the temperature and the real numerical aperture NA of an objective lens deviates from the designed value.
The PCA is an area on which test recording for an optimum laser power adjustment is carried out before an actual recording on the program area. The PCA consists of a count area and a test area each having a plurality of sub-areas. On each of the sub-areas of the count area, predetermined EFM signals are recorded under one-to-one correspondence to each corresponding sub-areas of the test area. On each of the sub-areas of the test area, the test recording are carried out, before the actual recording to the program area, so as to determine an optimum recording power of the recording laser. These areas are provided because the recommended recording power level is not always coincident with the optimum recording power level, as described above. In the test recording, one sub-area is used for one test recording and the sub-area once used for test recording becomes no longer recordable. Therefore, in the case of next test recording, it is necessary to search and find an un-recorded sub-area positioned inner circumferential side of the used sub-area. The count area is provided so as to easily find the un-recorded sub-area. That is, when a certain sub-area of the test area is used for the test recording, a predetermined EFM signal is recorded onto the corresponding sub-area of the count area. Accordingly, an unrecorded sub-area of the test area can be find out by searching and finding an unrecorded sub-area the count area.
The PMA is an area used for successively storing a history of the recording information on the program area. More concretely, the beginning and end addresses of each track number recorded on the program area, etc., are recorded in the PMA in the same recording format as the TOC information recorded on the lead-in area.
With respect to a partially recorded R-CD (hereinafter referred to as "partial disc"), TOC information cannot be recorded in the lead-in area until an end of recording finally instructed by a user since there is the possibility that information is subsequently written in the remaining program area. Therefore, it is necessary to provisional record information of respective tracks already recorded. This is why the PMA area is provided with the R-CD. On the other hand, when a recording end instruction signifying that no more information will be recorded is given by a user, TOC information and the lead out code are recorded, respectively.
Since the R-CD on which recording is finally completed (hereinafter referred to as "finalized disc") has a format in conformity with that of a compact disc, it can be reproduced as well as a compact disc by a compact disc player exclusive for reproduction which does not comprise the ATIP decoder, etc.
FIG. 4A shows an information recording condition of the virgin disc, FIG. 4B shows an information recording condition of the partial disc and FIG. 4C shows an information recording condition of the finalized disc.
However, if the partial disc is tried to be reproduced by the conventional CD player, no EFM signal is obtained the lead-in area thereof because no information has been recorded in the lead-in area of the partial disc. Therefore, the rotation of the spindle motor may become irregular and out of control of the spindle servo. In such a case, it is conceivable to perform the spindle servo control by way of detecting the wobbling frequency of the guide grooves on the partial disc. However, it is difficult because conventional CD players are not provided with means for detecting the wobbling frequency. Further, some types of the conventional players have such a construction that the pickup can not be moved to the position of the PMA, i.e., inner circumferential position of the lead-in area.